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Evaluating Gene Flow Using Selected Markers: a Case Study

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Evaluating Gene Flow Using Selected Markers: a Case Study Copyright 1998 by the Genetics Society of America Evaluating Gene Flow Using Selected Markers: A Case Study Thomas Lenormand, Thomas Guillemaud, Denis Bourguet1 and Michel Raymond Laboratoire GeÂneÂtique et Environnement, Institut des Sciences de l'Evolution (UMR 5554), Universite Montpellier II, 34095 Montpellier Cedex 5, France Manuscript received September 16, 1997 Accepted for publication March 9, 1998 ABSTRACT The extent to which an organism is locally adapted in an environmental pocket depends on the selection intensities inside and outside the pocket, on migration, and on the size of the pocket. When two or more loci are involved in this local adaptation, measuring their frequency gradients and their linkage disequilbria allows one to disentangle the forcesÐmigration and selectionÐacting on the system. We apply this method to the case of a local adaptation to organophosphate insecticides in the mosquito Culex pipiens pipiens in southern France. The study of two different resistance loci allowed us to estimate with support limits gene ¯ow as well as selection pressure on insecticide resistance and the ®tness costs associated with each locus. These estimates permit us to pinpoint the conditions for the maintenance of this pocket of adaptation as well as the effect of the interaction between the two resistance loci. LTHOUGH evolutionary theory attempts mainly to this ®nite variance requires knowledge of population A explain past changes, its predictions can be tested sizes and of the patterns of isolation by distance (Rous- by examining actual evolutionary processes in natural set 1997). populations. To do so we must quantify the determinis- Another approach is to analyze directly the relative tic processes causing genetic evolution, namely, selec- magnitude of gene ¯ow vs. selection through clinal pat- tion and gene ¯ow, and take into account the unpredict- terns that have been extensively studied theoretically for able changes due to stochastic processes such as random various selection models (Felsenstein 1976; Endler drift and mutation. Among these factors only gene ¯ow 1977). In an in®nite environment and at equilibrium, (and stabilizing selection) will oppose genetic differenti- the cline slope is a robust estimate of the relative mag- ation between populations. Its evaluation is therefore nitude of selection vs. migration (Barton and Gale required for the understanding of the evolution of pop- 1993). Different methods can be used to infer the abso- ulations in their ªadaptive landscapeº (for review, see lute value of each term, and in all cases, they require Slatkin 1987). However, methods are lacking to evalu- extra information about the system such as (1) a direct ate gene ¯ow independently of selection, mutation, or measure of dispersal, thus giving an indirect estimate drift. of selection (Endler 1977; Barton and Hewitt 1985); In most cases, it is possible to determine the relative (2) a genotypic parameter such as heterozygote de®- magnitude of gene ¯ow vs. drift, and thus estimate the ciency at one locus or linkage disequilibria when several degree of isolation of populations. This determination loci are involved, the latter being more reliable (Mal- enables the evaluation of the effects of different kinds let and Barton 1989); (3) the variation of the cline of selection, of the geographic scale of a local adaptation through time, for instance, the speed of a wave of ad- (Nagylaki 1975), or whether such populations might vance of an advantageous gene (Fisher 1937) or the be able to cross an ªadaptive valleyº (Lande 1985). rate of modi®cation of the cline shape when selection These methods, in principle, are valid for neutral and or migration is not constant. Despite their potential for independent genes at equilibrium and for a given rate the understanding of the evolution of populations over and mode of mutation. When averaged over many loci, their ªadaptive landscape,º these methods have mainly these methods may be robust to slight departures from been used for the analysis of tension zones (Barton the assumptions (Slatkin and Barton 1989). However, 1982; Szymura and Barton 1986; Mallet et al. 1990; they provide an estimate of the number of ªeffectiveº Sites et al. 1995). The aim of this article is to show that migrants but not of migration variance (s2). Estimating these methods can also be useful for understanding the dynamics of local adaptation. We have investigated the case of local adaptation of Corresponding author: Thomas Lenormand, Laboratoire GeÂneÂtique the mosquito Culex pipiens pipiens to organophosphate et Environnement, Institut des Sciences de l'Evolution (UMR 5554), insecticides in the Montpellier area in France. This ad- Universite Montpellier II, CC065 Place E. Bataillon, F-34095 Montpel- lier Cedex 5, France. E-mail: [email protected] aptation is conferred by resistance alleles at two major 1 Present address: Station de Recherche de lutte biologique, INRA La loci. Insecticide selection varies geographically, creating MinieÁre, 78285 Guyancourt Cedex, France. a pocket of adaptation. We have analyzed clinal patterns Genetics 149: 1383±1392 ( July 1998) 1384 T. Lenormand et al. at these two loci to estimate selection intensities and TABLE 1 gene ¯ow. These estimates were used to evaluate the Nomenclature role of interaction between the two loci for the main- tainance of the pocket of adaptation. Finally, we com- Coding rules pared our estimates to direct or indirect estimates of selection intensity and gene ¯ow in other studies. Genotype Genotype Phenotype Class Ester4 Ester4 (44) [4] {E} Ester4 Ester 0 (40) [4] {E} MATERIALS AND METHODS Ester 1 Ester 1 (11) [1] {E} Ester 1 Ester 0 (10) [1] {E} Culex pipiens and its environment: Larval development of 2 2 the mosquito C. p. pipiens takes place mainly in anthropic Ester Ester (22) [2] {E} 2 0 pools where insecticide control occurs. Females are presum- Ester Ester (20) [2] {E} ably fertilized at emergence (Weidhass et al. 1973) and then Ester4 Ester 1 (41) [41] {E} search for their blood meal and a site to lay z150±200 eggs. Ester4 Ester 2 (42) [42] {E} Insecticides are applied during the breeding season, that is, Ester2 Ester 1 (21) [21] {E} approximately from April to October near Montpellier in Ester 0 Ester 0 (00) [0] {O} Chevillon southern France (see et al. 1995 fordetails) andare Ace.1R Ace.1R (RR) [RR] {R} restricted to a 20-km coastal belt. Between 1968 and 1990±91, Ace.1R Ace.1S (RS) [RS] {R} organophosphate (OP) insecticides were exclusively used for Ace.1RS Ace.1S (RSS) [RS] {R} mosquito control and have been replaced since then by the Ace.1RS Ace.1R (RSR) [RS] {R} Bacillus sphaericus toxin. However, in the Montpellier area, OP Ace.1RS Ace.1RS (RSRS) [RS] {R} insecticides are still used at large doses to control other Culic- S S ids and even to control C. p. pipiens in some situations (Anony- Ace.1 Ace.1 (SS) [SS] {S} mous 1990±1995). Additionally, residual doses of OP may be The resistance allele Ester 1 corresponds to overproduced of the order of lethal concentrations for susceptible mosqui- esterase A1, and alleles Ester 2 and Ester4 correspond to overpro- toes in many places of the treated area, possibly due to other S R. Eritja, duced esterases A2-B2 and A4-B4, respectively. Ace.1 codes pest controls ( personal communication). for a sensitive AChE1, Ace.1R for an insensitive AChE1, and Genetics of resistance: Two main loci are responsible for Ace.1RS for both acetylcholinesterases. ªCoding rulesº indicates OP resistance in C. p. pipiens. The ®rst locus, Ace.1, codes for Bourguet the simpli®ed code for each genotype, phenotype, and class an acetylcholinesterase (AChE1), the OP target ( corresponding to the genotype indicated in the ®rst column. et al. 1996a; Malcolm et al. 1998) and has three alleles: Ace.1R S ªPhenotypeº corresponds to the electrophoretic phenotype that codes for an insensitive AChE1; Ace.1 that codes for a (Ester locus) or to the TPP phenotype (Ace.1 locus). ªClassº sensitive AChE1; and Ace.1RS that corresponds to a duplication groups genotypes with at least one resistance allele. {E}/{O} of and codes for both enzymes (Raymond 1986; Ace.1 et al. for the presence/absence of at least one overproduced ester- Bourguet 1996b,c). The second locus corresponds to et al. ase and {R}/{S} for the presence/absence of at least one insen- a ªsuper locus,º that is, two closely linked loci, and Est-3 sitive AChE1. Est-2, coding for esterases A and B, respectively (de Stordeur 1976; Pasteur et al. 1981a,b). Only 2 to 6 kb of DNA separate Est-3 from Est-2 (Rooker et al. 1996; Guillemaud et al. 1997). Resistance alleles at Est-3 and Est-2 induce an overproduction populations, resistance alleles at both loci are associated with of esterase, resulting in gene ampli®cation or gene regulation ®tness costs, in the absence of insecticides, through decreases (Rooker et al. 1996). Due to their close proximity, esterase in fecundity and adult survival and an increase in larval devel- genes are often coampli®ed as a single unit, which explains opmental time. These ®tness costs tend to be higher for insen- the complete association of resistance alleles at both loci sitive acetylcholinesterase than for overproduced esterases (Rooker et al. 1996; Guillemaud et al. 1997). This coampli®- (Chevillon et al. 1997). cation justi®es considering them as a single ªsuper locus,º Data collection: Pupae were sampled on July 5, 1995 in 10 which will hereafter be designated as Ester. In southern France, breeding sites along a 50-km north-south transect (Figure 1) three resistance alleles have been identi®ed at this locus. Ester 1 across the treated and untreated areas studied by Guillemaud corresponds to an increased expression of the esterase A1, et al.
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